Magnetic amplifier system



Feb. -8, 1949. A. s. FITZ GERALD MAGNETIC AMPLIFIER SYSTEM Filed May 3, 1946 3 Him ATTORNEY Feb. 8, 1949. A. s. FITZ GERALD 2,461,046

MAGNETIC AMPLIFIER SYSTEM Filed May 3, 1946 2 Sheets-Sheet 2 13/ 7 Jiiy 6 m N N m v IN VENTOR ALAN S. F/TZ GERALD ATTORNEY 15 11 V \i A k V m K Patented Feb. 8, 1949 UNITED STATES PATENT OFFICE MAGNETIC AMPLIFIER SYSTEM Alan S. Fitz Gerald, Wynnewood, Pa.

Application May 3, 1946, Serial No. 666,867

13 Claims. 1

This invention relates to variable electro-magnetic devices such as reactance bridges and the like, and more particularly to magnetic amplifiers of the type which employ saturating reactors.

In electrical systems of the type referred to, there are commonly included devices, the reactance of which may be caused to vary, in response to an external influence, so that the currents flowing therein change from one magnitude to another, neither of which may approach zero. It is accordingly an object of this invention to provide circuit arrangements whereby there may be derived from such a system an output which may be caused to vary, in response to an external influence, between two magnitudes, one of which may, if desired, be substantially zero. It is a further object of my invention to provide an improved magnetic amplifier system based on the above principles.

One way in which the above result has been accomplished in magnetic amplifier systems previously known in the art, has involved the use of duplicate saturating reactors balanced one against the other. In some instances, a single saturating reactor is used in conjunction with an inactive, dummy, or counterpoise reactor, having similar magnetic characteristics. Outputs are derived from such arrangements in accordance with the difference between the magnetic condition of the two cores. According to such arrangements, this differential action is usually obtained by means of a transformer. Magnetic amplifiers are commonly associated with very low power levels, for example, input values of the order of less than one-thousandth of a microwatt are representative. Thus, thepower levels of the outputs of the first and second stages of a multi-stage magnetic amplifier may still represent relatively low magnitudes. It is well known to those skilled in the art that efiicient transformation of very low power levels is not very easily attained. It is, therefore, a further object of the present arrangement to avoid inter-stage transformation.

According to another method of accomplishing the result referred to, previously utilized by the present inventor, additional neutralizing or compensating windings have been employed in devices or further stages receiving power from one stage of a magnetic amplifier, in order to furnish excitation equal and opposite to that which is due to the output received from the said stage when the input thereto is zero.

When the output of such a-magnetic amplifier is intended to operate a control device such as a relay or the like, it has been found that there is considerable element of inconvenience in furnishing such an additional compensating coil, inasmuch as this involves the provision of special windings. Standard relays cannot be employed in the form in which they are manufactured and stocked. Furthermore, a supply for energizing such bucking windings may be provided, involving additional structure and material for this purp se.

In cases where the output of a magnetic amplifier is to be received by a further magnetic amplifier stage, the use of such a compensating winding is attended by two disadvantages. Particularly in the case of a last stage, or one several stages removed from the input stage, that is to say, a stage designed to operate with a substantial input power level, as for example, milliwatts rather than microwatts, high permeability core material would not normally be used, but rather material of normal characteristics. Such core material is characterized by the fact that the output current of a saturating reactor, when there is no effective input to the D. C. saturating winding, is relatively high in comparison with the output when the input excitation is present; in other words, the proportional change in current magnitude, due to the input, is not great. When such conditions are present in a magnetic amplifier, substantial amounts of compensating ampere-turns are required, as a result of which the compensating winding may occupy as much as one-half of the window area available in the saturating reactor core for the D. C, saturating excitation. It will be obvious to those skilled in the art that'the greater the percentage of the window-area that may be occupied by the active input saturating winding, the greater will be the ampere turns furnished by a given input power level; and, therefore, the greater will be the gain.

Another and more important disadvantage attending the use of the compensating winding principle is that this winding has to be energized through a separate rectifier. Thus, the performance of the magnetic amplifier is dependent upon the relation between two currents, one in the input winding and one in the compensating winding, which relationship is a function of the characteristics of two different rectifiers. It is well known to those skilled in the art that rectifiers are subject to some limitations in respect of uniformity and constancy of characteristics, and have also marked non-linear properties, as a result of which the relationship between the two currents may be disturbed by system voltage variations. Likewise, eflects resulting from am- 3 bient and apparatus temperature change produce similar discrepancies.

It is accordingly another object of my invention to provide an improved magnetic amplifier system less affected than previous arrangements by system voltage variations or by temperature eiiects.

It is a further object of my invention to provide arrangements of the type referred to which can be utilized to energize any existing relay or other electro-responsive device, of suitable resistance value, without the rovision of special windings.

It is yet another object of my invention to provide an arrangement for furnishing an electrical output, variable from zero, in accordance with a mechanical displacement or movement.

In previously known types of magnetic amplifiers which require duplicate or counterpoise reactors, it is usually necessary that these have similar electrical and magnetic characteristics. saturating reactors employed in the input and intermediate stages of a sensitive magnetic amplifier usually have cores of high permeability magnetic material of which the cost is relatively high.

It is, therefore, yet another object of my invention to provide a magnetic amplifier of the type referred to requiring less material than heretofore, resulting in decreased bulk and cost.

These and other novel features which I believe to be characteristic of my invention will be set forth with particularity in the appended claims. My invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood with reference to the following description, taken in connection with the accompanying drawings, in which:

Fig. 1 is an electrical circuit diagram showing a single-stage magnetic amplifier in accordance with a preferred embodiment of my invention;

Figs. 2 and 3 are diagrammatic views in front Lil elevation of two forms of-construction of saturating reactors which may be used in carrying my invention into eflect;

Fig. 4 is a fragmentary portion of the Fig. 1 circuit, illustrating a modified type of variable reactor which may be employed therein;

Fig. 5 is a further fragmentary showing of the Fig. 1 circuit, illustrating an additional modification thereof for incorporating an additional direct current winding which may be employed as a biasing winding or as a regenerative feed-back winding as explained below;

Fig. 6 is a circuit diagram representing a twostage magnetic amplifier based upon the arrange-' ment shown in Fig. 1; ,while Figs. '7 and 8 are circuit diagrams representing two different arrangements for incorporating a regenerative or feed-back efiect in the basic circuit of Fig. 1, together with the feature of selective response to inputs of diiferentpolarity.

Referring to the drawings wherein like elements are similarly designated, I show in Fig. 1 a saturating reactor having a. magnetic core I, an alternating current or reactance winding 2, and a direct current, saturating winding 3. The structural configuration of the core I and the dis position thereon of the several windings may be in to be particularly satisfactory for the practicing of my invention.

As shown in these figures, the alternating cur rent or reactance winding 2 comprises two sections arranged on different limbs of the magnetic core I. The two sections of the winding 2 may be connected either in series or in parallel. In the figures referred to, I have shown them connected in series. The reactance winding 2 is disposed on the two aforementioned limbs so that no voltage is induced by them in the saturating winding 3. Thus, in Fig. 2, the core I has three limbs, the A. C. winding 2 being equally divided between the two outer limbs, the D. C. or saturating winding 3 being disposed on the center limb. In Fig. 3, the core has four limbs and the two sections of the A. C. winding 2 are mounted on the two middle limbs. The D. C. winding 3 in Fig. 3 embraces also the two middle limbs as shown, surrounding the two sections of the A. C. winding 2.

In both Figs. 2 and 3, the two sections of the A. C. winding 2 are connected in opposite senses respectively with reference to the D. C. winding 3 so that, as required, there is no inductive relation or transformer action between the windings 2 and 3.

The magnetic amplifier circuit of Fig. 1 contains, in addition to the saturable core reactor I, 2, 3, a transformer consisting of a magnetic core 4, carrying primary and secondary windings 5 and B. The circuit also contains a pair of impedances which preferably take the form of resistances I and 8. The primary winding 5 of the transformer is connected in series with one of these resistances I, between a. pair of conductors Ii, I2. Similarly, the A. C. winding 2 of the satu rable reactor is connected in series with the other resistance 8, between conductors ll, I2. Conductors II, I2 are, in turn, connected to a pair of terminals I3 for energizing the circuit elements above recited from an alternating current source, as is indicated in the drawings.

The voltage of the source l3 should be somewhat higher than the A. C. voltage which, when applied directly to the A. C. winding 2 of the saturable core reactor, would cause appreciable saturation of core I thereof. For example, I have found that a convenient arrangement is to employ a supply voltage of the order of 25 volts, such as is commonly used in connection with heat control circuits, and to provide the winding 2 with a suitable number of turns such that it approaches maximum permeability with a voltage in the neighborhood of 5.

With the circuit of Fig. 1 energized from an alternating current source at I3, as above describ ed, output power may be derived therefrom over conductors 9, I0, connected respectively to opposite sides of the A. C. winding 2 of the satul rable reactor, conductor 9 being directly so connected, while conductor III is thus connected through the secondary winding 6 of the transformer 4, 5, 6.

In the preferred arrangement of the circuit, the polarity of the secondary winding 6 in connection I0, is such that the instantaneous voltage induced therein from the primary winding 5 cpposes the instantaneous voltage drop across the A. C. winding 2 of the saturable reactor, whereby, by appropriate selection of the magnitudes for resistance elements 1 and 8, the output voltage developedbetween conductors 9 and III may be reduced substantially to zero, or to any other desired value, for zero current in the direct ourand in will likewise progressively increase fromthe preselected initial or zero value aforesaid. The output voltage developed between conductors I and I is, of course, alternating in character, but may be converted to direct current by the inclusion in the output circuitof a rectifier ll,

from which extend direct current output leads ll.

The circuit of Fig. 1 is thus, in effect, an alternating current bridge circuit, which is balanced for a particular condition of the saturable core reactor i, 2, 3, preferably for the condition when no direct current is traversing the saturating winding 3. Considering the circuit from this standpoint, the primary winding of the transformer and the A. C. winding 2 of the saturable reactor constitute one pair of adjacent balancing arms; while the resistances 1 and 8 constitute the remaining pair of adjacent balancing arms. The A. C. supply circuit II, i2, i3 constitutes one conjugate arm; while the output circuit 9, l0 constitutes the remaining conjugate arm, whereby, for the condition of balance of the bridge aforesaid, the alternating current impressed on the conjugate arm ll, I2, It produces substantially no effect or output in the remaining conjugate arm 9, l0.

Considering the Fig. 1 circuit from a somewhat different aspect, its operation may be explained in a different manner, by first considering the action which takes place without the presence of the resistance 8, and with the resistance 1 capable of adjustment. It will be seen that, in the absence of the resistance 8, the output circuit i5 derives power from the A. C. source through a channel which comprises the primary winding 5 of the transformer, the resistance 1, the secondary winding 6, the A. C. winding 2 of the saturable reactor, and the rectifier ll. Under this condition, even though there be no D. C. saturating current applied to the input winding 3, current will flow in the output circuit 9 commensurate with the magnetizing current drawn by the winding 2 in accordance with the value of the resistance of 1 and the turn-ration of transformer windings -5 and 6. In a number or arrangements which I have studied, it has been found that the number of turns which should be wound on the secondary winding 6 varies from approximately one-third to four-fifths of the turns provided on the primary winding 5.

If, now, a relatively high resistance, also adjustable, be inserted where indicated in the diagram at 8, and with the correct polarity connections of the primary and secondary of the transformer as aforesaid, it will be found that the current in the output circuit It will be reduced; and that, with suitable circuit constants, with particular reference to the transformer turns ratio and the values of resistances 1 and 8, it is possible to arrive at values for the latter resistances at which the current in the output circuit I5 is reduced substantially to zero. By the expression substantially to zero, I mean that the value may be reduced to a figure of the order of one per cent of the value which would flow in the absence of the resistance 8.

Current values of this order of magnitude are without discernible effect with respect to the input of a succeeding stage and are therefore, for practical purposes, the equivalent of zero. If,

now, an input current be caused to flow in the I). C. saturating winding 3, output current will appear in the output circuit 9, II, which output current will increase with increase in the magnitude of direct current traversing the saturating winding 3.

In determining the proper values of the resistances 1 and 8, because of the vector relations which are present in this circuit, when these resistances are approximately, but insufficiently. near to the correct value, the current in I, II will be found to decrease to a clearly defined minimum other than zero. The optimum values which result in substantially zero current can, however, readily be determined by further trial.

A number of different values for the resistances 1 and 8 can be found which will produce zero noload current in the output circuit. However, in view of the fact that, when an input current is supplied to the winding 3. energization of the output circuit 9, II! is derived through the resistance I, it is desirable to employ as low a value for 1 as will meet the required circuit conditions, with particular reference to the vector relations.

I have found that, in practicing my invention, it is desirable to have rather fewer turns'for the primary winding 5 than would normally be used for a transformer operating at the voltages which exist. The current flowing in the resistance 1, when the circuit is properly balanced, is the magnetizing current of the transformer. Since a component of the required vector relations is an A. C. voltage proportional to the product of the resistance of 1 and the current flowing therein, it is clear that the higher'the magnetizing current of the transformer the lower may be the value of resistance at which the desired voltage is produced.

While I have shown in Fig. l, by way of example, an embodiment of my invention including a variable reactance device in the form of a saturating reactor, in which the reactance is varied by a D. C. saturating input current, I wish it to be clearly understood that the core I and the reactance winding 2 may consist of any other suitable variable impedance device without departing from the spirit of my invention. It will be understood that the term variable impedance device is intended to refer to any device, the electrical impedance of which is variable in accordance with a controlling effect.

For example, in Fig. 4, I show a portion of the circuit of Fig. 1 in which the variable reactance device is represented in the form of a movable core arrangement such as for example, a solenoid I, 2. I employ like numerals to indicate the magnetic core 1 and the A. C. reactance winding 2. Portions of the circuit not shown in Fig. 4 are to be understood to be identical with the arrangement of Fig. 1.

With the arrangement of Fig. 4, the adjustment of the circuit constants may be made in the manner already described in reference to Fig. 1. For example, suppose that the core I has a normally position, in which it is fully entrant into, and embraced by the A. C. winding 2, and that when actedv upon by an external effect or influence, it becomes partially retracted therefrom. When the core is in its normal position, the winding 2 will have a reactance of a predetermined value. When the core I is retracted, the reactance will decrease. in accordance with the extent to which the core is withdrawn. Accordingly, the values of 1 and 8 may be so adjusted when the core i is in the normal position; the output to the load circuit 9 is zero. When the core I is retracted, commensurate output current, will flow in the circuit 9.

I have found the Fig. 4 arrangement to be pletely out.

Although I have indicated in the foregoing description that adjustment of the resistances I and 8 may be made such as to furnish zero output with a fully entrant normal position of the core i, it will be apparent that, equally readily,

for other purposes, as for example when a crossdimming action is desired, the resistances i and ,8 may be adjusted to give the opposite effect, that 'is to say, the output may be zero when the core I is retracted and may be caused to increase in accordance with the extent to which the core is pushed in.

In some cases, particularly where it is desired ..-to--provide a magnetic amplifier of high sensitivity, responsive to very low inputs, improved results may be obtained by separately exciting the core I of the saturating reactor with a constant and predetermined value of direct current excitation such that the core i is caused to operate at or near to the flux-density at which maximum permeability occurs.

In Fig. 5, I show a portion of the circuit of Fig. 1 in which the saturating reactor is provided with an additional bias winding l6 for this purpose. The winding i6 is energized with direct current excitation, usually of a low order of magnitude, of polarity such as is required to furnish the desired magnetic" conditions, from any convenient source of direct current, as for example, from a rectifier energized from the alternating current source.

The position of the bias winding ii on the core I of the saturating reactor is similar to that of the input winding 3, as indicated by the corresponding numerals in Figs. 2 and 3.

. .In Fig. 6, I show an application of the circuit or-Fig. l to multi-stage magnetic amplification, the circuit of Fig. 6 comprising a two-stage amplifier of this type. Like elements are similarly designated in Figs. 1 and 6, and it will be observed therefrom that each stage of the multistage amplifier, Fig. 6, is identical with the circuit of Fig. 1. It will also be observed that, in the multi-stage amplifier of Fig. 6, the rectified output of the first stage is applied over conductors I5 to the direct current saturating winding 3 of the saturable reactor I, 2, 3 of the second stage, the same arrangement. being employed for any number of additional stages that might be desired in a particular instance.

An advantage of my invention, which is particularly brought out in connection with multistage magnetic amplifiers, is that each stage may be a separate and self-contained unit. That is to say, any given stage arranged according to the present invention may be used, if desired, as an initial stage, as an intermediate stage, or as a final stage, without the necessity of providing additional windings for the latter applies) tions. All that is necessary is that the output of a first stage be directly connected to the input of a second stage and that the resistance of the second stage be suitably matched in reference to the circuit constants of the previous stage.

This characteristic of the present invention, in thateach stage is a completely separate entity, as compared with arrangements of the prior art which use additional compensating windings, is of additional convenience in setting up and adjusting multi-stage circuits. This arises from the fact that the adjustment of each stage is likewise entirely internal and independent of other preceding or succeeding stages. All inter-stage currents may be adjusted to zero, which condition is simpler and more readily arrived at than in the case of the previously mentioned prior arrangements in which the inter-stage currents are never, under any circumstances, zero, and adjustments have to be made in one stage, which are dependent upon the magnetic characteristics of the preceding stage, which adjustments involve the location of a definite minimum rather than a null point.

Although I have indicated above the zero or null adjustment of the inter-stage currents may be made, I desire to point out that in some instances the gain may be improved by adjusting the resistances i and 8, so that the balance of the circuit is slightly oifset from zero, in the direction which is additive in relation to the effect produced by applying input to the winding, 3. In general, adjustments made in this manner may be beneficially employed to an extent which is limited by the ability of the circuit completely to react, that is to say, to be restored to its initial condition when the input is withdrawn.

The characteristics of the magnetic amplifier arrangements so far described in this specification are such that the response is of the type known to those skilled in the art as neutral"; that is to say, the output produced is not affected by the polarity of the D. C. input excitation to the saturating coil 3. By including in the circuit of Fig. 1 a suitable amount of regeneration, or feed-back, a number of additional characteristics may be obtained. For example, greatly increased gain and sensitivity results; furthermore, the response is no longer of the neutral type but, instead, the output of the magnetic amplifier depends upon the polarity of the input.

, In addition, several qualitatively different types of action result in accordance with the quantitative extent to which feed-back is introduced.

I With moderate amounts of feed-back, the response of the magnetic amplifier to an input is stable and corresponds in character with the action obtained in the absence of teed-back; if the connections are made such that the feed-back is positive, with any given input polarity, the output is increased by the feed-back, if the feedback is negative, the output is decreased. Thus, with any given feed-back connections, the output is greater with input of one polarity and less if the polarity be reversed. If desired, the values of the resistances I and 8 may be adjusted so that with no input, a finite amount of output current results, in which case this current will be increased by input of one polarity and decreased by input of another polarity. .Provided the amount of feed-back does not exceed amounts such that stable action is produced, the conditions preceding the applications of an input signal will be restored when the signal is withdrawn.

However, if substantially greater feed-back is employed, the magnetic amplifier circuit will have two distinct conditions of stability and, in general, will act in the manner which I have described in my U. S. Patent #2,027,312, and which it is not necessary to particularize at length in the present specification.

Briefly, the action is analogous to that of a "latched in type of electric relay. One condition of stability gives a high current and one other gives a low current. Input of one polarity, which we may conveniently refer to as positive, causes the circuit to assume the first mentioned condition, which condition is retained when the signal is withdrawn. A negative signal produces the opposite effect causing the circuit to revert to the low-current condition. Again, this low-current condition is retained indefinitely after the signal is withdrawn.

A third and especially valuable type of action may be provided by employing suitable circuit constants, which result in an amount of feedback intermediate between those which produce the two aforementioned different types of action. By making the appropriate adjustments, a floating type of response may be obtained. This type of action is of great value in connection with automatic control systems of the self-balancing bridge or so-called closed-cycle type. Its pertinence to this type of control system is best understood by considering the type of automatic control which embodies a reversing motor operating, through a train of gears, a variable output device, such as a rheostat, induction regulator or the like. According to such arrangements, by means of a relay or analogous responsive device, whenever the control system departs from the normal condition, according to the sense of the deviation, the motor is caused to rotate in one direction or the other, as required, to restore the system to the normal condition.

The use of a gear ratio, such as gives relatively slow rate of corrective response, enables sensitive relay systems to be used without overshooting or hunting, as is well known to those skilled in the art.

It is a characteristic of such arrangements that when the normal condition is restored. the

variable output device remains stationary for an indefinite period until such time as a further deviation occurs, and that the variable output device may come to rest at any position within its range of motion.

In the past, this type of action has been exclusively associated with electro-mechanical apparatus involving such elements as sliding contacts and/or movable electro-mechanical parts.

The circuit of my present invention, when adjusted for this floating action, furnishes an analogous type of control over limited periods of time of the order of seconds and minutes. When utilized in conjunction with a continuously operating automatic self-balancing system, in which deviations from a normal condition are self-correcting, for practical purposes, action of this type over relatively short periods of time is just as effective as would be the case if apparatus be used of such characteristics that a given condition may be indefinitely prolonged.

The nature of this floating action may best be described by considering that at a given time the output of the magnetic amplifier has an observed value and that a small amount of positive excita- 10 tion be applied to the input. The output of the amplifier will be seen to commence increasing. If the input be maintained, the output current will continue to increase until it reaches an upper limit.

If the positive input be withdrawn, the output will cease to increase at the time of interruption of the input and will remain at approximately the value at which the input signal was withdrawn. Over more prolonged periods, it will slowly drift away from this value. If, on the other hand, a low negative input be applied, the output will, in like manner, be steadily decreased as long as the negative signal is maintained. Likewise, it will tend to remain at or around any value which exists when the signal is interrupted.

It will be obvious from the above description that if manual means for applying, withdrawing and reversing an input signal be furnished, the output current may be manually held at any given value by applying, from time to time, momentary corrective signals of positive or negative value, according to the direction of deviation. Obviously, this same type of action may be utilized in automatic instead of manually controlled arrangements.

Feed-back may be introduced into the arrangement, according to Fig. 1, in several different ways, two of which are representatively illustrated in Figs. 7 and 8.

In Fig. '7, I show an additional winding [6 situated on the saturating reactor core in a position corresponding to that of the D. C. winding IS in Figs. 2 and 3, and connected in series with the rectified output circuit [5 of the magnetic amplifier.

In Fig. 8, I show a. resistance I1 connected in series with the D. C. input winding 3 of the saturable reactor, and also connected in series with the rectified output circuit l5 of the magnetic amplifier.

If desired, both of these arrangements may be employed conjointly. Each of the two different arrangements of Figs. 7 and 8 has specific advantages for different operating conditions which it may be of interest to point out.

In Fig. '7, the feed-back circuit is separate from the input circuit and may be changed or adjusted without altering the resistance or circuit constants of the input circuit. However, the arrangement shown in Fig. '7 involves special structure incorporated in the saturating reactor, in that the additional winding l6 has to be especially provided. On the other hand, according to the arrangement shown in Fig. 8, feedback can be introduced into any existing amplifier having only a single D. C. winding, merely by the addition of the resistance i1. However, since the resistance I1 is included in the input circuit, adjustments in the value of this resistance may affect the circuit arrangement and where this is not desired the arrangement shown in Fig. 7 may be preferred.

In order better to illustrate the action of this circuit, I give below some typical numerical values taken from a practical embodiment of my invention which I have constructed and tested. It should, however, be clearly understood that my invention may be carried into effect on any desired scale of magnitude and may be modified in any manner conformable with the purpose and application for which it is to be employed. I am, therefore, in no way to be limited by the following data which is included only for the purpose of facilitating the understanding of my invention.

In the aforementioned embodiment, intended for an initial stage of moderate sensitivity, or for a second stage, the saturable core reactor i, 2, 3 had a core i consisting of approximately 0.69" stacking of three-limb laminations of proportions similar to that shown in Fig. 2 having over all dimensions approximately 2.63 x 1.94". The center limb was 0.69" wide and the two outside limbs were -0.38" wide. The stamping had a window area of 0.71 square inches. The supply voltage was approximately 25 volts, 60 cycles. The laminations were made from a nickel-iron alloy containing 50 per cent or less of nickel. Each section of the winding 2 consisted of 150 turns. The winding 3 comprised 5300 turns and had a resistance of 400 ohms.

The core 4 of the transformer I, 5, 6 was constructed of 0.63" stacking of conventional transformer laminations 1.88" long by 1.56" wide. The primary 5 comprised 320 turns, and the secondary winding 6 had 110 turns.

The resistance of 1 was 500 ohms and that of 8 was 2800 ohms. The rectifier II) was of the copper oxide type having one plate in series in each of the four legs of the bridge, the plates being A of an inch in diameter.

With this arrangement, an input current of approximately 275 microamps representing an input power level of 30 microwatts produced an output current of the order of 3 milliamps flowing in a load circuit having a resistance'of 500 ohms, which latter value is close to the optimum for the circuit constants and input power level given. This corresponds to an output power level of 4.5 milliwatts representing an input-output ratio approaching 150.

For an input stage which it is desired be responsive to power levels substantially less in magnitude than the above, I may use still more sensitive core material, as for example, a material known to the industry as mu-metal, consisting of a nickel-iron alloy containing more than 70 per cent nickel. Such an input stage may be capable of responding to input power levels of the order of small fractions of a microwatt. Suitable and different values of the windings and resistances are appropriate where this core material is used.

For stages associated with still higher power levels, such as I have employed in the last stage in a magnetic amplifier for furnishing power directly to a relay, motor or the like, I employ laminations of regular transformer coil material.

In such a stage, I prefer to use a core of the type f shown in Fig. 3 which has four limbs instead of three. I have found that the form of core shown inFig. 2 is preferable where the power level is relatively low, but that the arrangement according to Fig. 3 is advantageous where substantially) higher power levels are involved.

With reference to the transformer, it has been noted that the operation of the invention is dependent upon the provision of the correct phase relations. Thus, for any given saturating reactor, the transformer should have conformable constants. That is to say, there is a correct and proper number of turns desirable for both the primary and secondary winding, and, likewise, the core should have suitable magnetic characteristics.

For example, I have found that in using an arrangement according to Fig, 4, it is sometimes an advantage if the transformer coil 4, be provided with a quite small air gap, such as may be 0bwinding 5.

In order to avoid the use of resistances of adjustable value, I may, if desired, provide resistances 1 and 8 of the nearest standardized resistor values and, in order to secure the desired operating condition, means for adjusting the characteristics of the core 4, as for example, varying the number of laminations, providing slidable mechanical means for adjusting the extent to which the laminations are interleaved; or by any other suitable means of adjusting the magnetic reluctance.

Although I have chosen a particular embodiment of my invention for the purpose of explanation, many modifications thereof will be apparent to those skilled in the art to which it pertains. My invention, therefore, is not to be limited except in so far as is necessitated by the prior art and the spirit of the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A magnetic amplifier comprising, in combination: a variable reactance device; a transformer having primary and secondary windings; said variable reactance device and said transformer primary being serially connected in-a closed circuit containing a pair of additional impedances; an alternating current energizing circuit extending from a point between said reactance device and said transformer primary to a point between said additional impedances; and an output circuit including said variable reactance device and said transformer secondary in series.

2. A magnetic amplifier comprising, in combination: a magnetic core, variable reactance device; a magnetic core transformer having transformer primary and secondary windings; a bridge circuit including said variable reactance device and said transformer primary as adjacent balancing arms, and a pair of impedances as the remaining balancing arms; a first conjugate arm of said bridge extending from a point between said impedances to a point between said reactance device and said transformer primary; and a second conjugate arm of said bridge being connected across said variable reactance device and including said transformer secondary.

3. A magnetic amplifier comprising, in combination: a variable reactance device; a transformer having primary and secondary windings; a bridge circuit containing said device and said transformer primary as adjacent balancing arms, and a pair of additional impedances as the remaining balancing arms; a first conjugate bridge arm extending from a point betwen said impedances to a point between said device and said transformer primary; a second conjugate bridge arm connected across said device and including said transformer secondary; said bridge elements being proportioned and arranged substantially to balance the bridge for a preselected adjustment of said variable reactance device.

4. A magnetic amplifier comprising, in combination: a saturable, magnetic core device having alternating and direct current windings; a transformer having primary and secondary windings; a bridge circuit containing said alternating winding and said transformer primary as adjacent arms, and a pair of additional impedances as the remaining arms; an alternating current 13 input circuit extending from a point between said impedances to a point between said alternating winding and said transformer primary; and an output circuit connected across said alternatin winding and including said transformer secondn.

5. A magnetic amplifier comprising, in combination: a saturable, magnetic core device having alternating and direct current windings; a magnetic core transformer having primary and secondary windings; a bridge circuit containing said alternating winding and said transformer primary as adjacent arms, and a pair of additional impedances as the remaining arms; an

alternating current input circuit extending from a point between said impedances to a point between said alternating winding and said transformer primary; and an output circuit connected across said alternating winding and including said transformer secondary; the elements comprising said bridge circuit being so proportioned and arranged as to render said input and output circuits substantially conjugate for zero direct current core saturation of said saturable core device.

6. A magnetic amplifier comprising, in combination: a saturating core reactor, a transformer having primary and secondary windings, a bridge circuit including said reactor and said transformer primary as adjacent balancing arms, and containing a pair of additional impedances as the remaining balancing arms, one conjugate arm of said bridge extending from a point between said impedances to a point between said reactor and said transformer primary, the remaining conjugate arm of said bridge including said reactor and said transformer secondary winding in series.

7. A magnetic amplifier comprising: a variable reactance device, means connecting said device in series with a first impedance across an alternating current source, a transformer having primary and secondary windings, means connecting said primary winding in series with a second impedance across an alternating current source, and an output circuit containing said variable reactance device and said transformer secondary winding in series.

8. A magnetic amplifier comprising: a saturable core device having alternatingand direct current windings, means connecting said alternating current winding in series with a first impedance across an alternating current source, a transformer having primary and secondary windings, means connecting said primary winding in series with a second impedance across said alternating current source, and an output circuit containingin series, said saturable core alternating current winding, and said transformer secondary winding.

9. A magnetic amplifier comprising: a saturable core device having alternating and direct current windings, means connecting said alternating current winding in series with a first impedance across an alternating curent source. a transformer having primary and secondary windings. means connecting said primary winding in series with a second impedance across said alternating current source, and an output circuit containing in series, said saturable core alternating current winding, said transformer secondary winding, and a rectifier.

10. A multi-stage magnetic amplifier comprising: a first stage containing a saturable core device having direct and alternating current windings, means connecting said alternating current winding in series with a first impedance across an alternating curent source, a transformer having primary and secondary windings, means connecting said primary winding in series with a second impedance across an alternating current source, and an output circuit including said saturable core alternating current winding and said transformer secondary winding in series, a second stage of amplification identical with the first, and a rectifier interposed between the output circuit of said first stage and the direct current winding of the saturable core device in the second stage.

1. A magnetic amplifier comprising: a saturahle core device having alternating and direct current windings, means connecting said alternating current winding in series with a first impedance across an alternating current source, a transformer having primary and secondary windings, means connecting said primary winding in series with a second impedance across said alternating current source, an output circuit containing, in series, said saturable core alternating current winding, said transformer secondary winding, and a rectifier, and means for applying rectified current derived from said output rectifier, to a direct current winding of said saturable core device.

12. A magnetic amplifier comprising: a saturable core device having an alternating current reactance winding and a direct current saturating winding, means connecting said alternating current winding in series with a first impedance across analternating current source, a transformer having primary and secondary windings, means connecting said primary winding in series with asecond impedance across said alternating current source. an output circuit containing, in series, said saturable core alternating current winding, said transformer secondary winding, and a rectifier, and a coupling impedance common to the output of said rectifier and to said direct current saturating winding.

13. A magnetic amplifier comprising: a saturable core device having an alternating current reactance winding and a plurality of direct current saturating windings, means connecting said alternating current winding in series with a first impedance across an alternating current source, a transformer having primary and secondary windings, means connecting said primary winding in series with a second impedance across said alternating current source, an output circuit containing, in series, said saturable core alternating current winding, said transformer secondary winding, and a rectifier, and a circuit extending from the output of said rectifier to said saturable core device, said circuit including a direct current saturating winding thereof.

ALAN 8. FITZ GERALD.

REFERENCES CITED The following references are of record in the file of this patent:

- UNITED STATES PATENTS Number Name Date 1,544.381 Elmen June 20, 1925 1,824.5'7'1 Sorewsen Sept. 22, 1931 1.965.439 Stoliek- July 3, 1934 2,388,070 Middel Oct. 30, 1945 

